U.S. patent number 4,642,688 [Application Number 06/736,301] was granted by the patent office on 1987-02-10 for method and apparatus for creating encrypted and decrypted television signals.
This patent grant is currently assigned to Scientific Atlanta, Inc.. Invention is credited to John D. Lowry, Keith Lucas.
United States Patent |
4,642,688 |
Lowry , et al. |
February 10, 1987 |
Method and apparatus for creating encrypted and decrypted
television signals
Abstract
A method and apparatus for creating a television signal and
encrypting or decrypting the signal at the same time. Luminance and
chrominance information are received by the apparatus and stored in
separate television scan line stores. The stored liminance and
chrominance information is read out from their respective stores at
a frequency corresponding to a desired format or standard to create
the television signal. The signal may be simultaneously encrypted
or decrypted by delaying the time at which the luminance and/or
chrominance information is read out in accordance with an
encryption or decryption key.
Inventors: |
Lowry; John D. (Toronto,
CA), Lucas; Keith (Oak Ridges, CA) |
Assignee: |
Scientific Atlanta, Inc.
(Atlanta, GA)
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Family
ID: |
24959344 |
Appl.
No.: |
06/736,301 |
Filed: |
May 21, 1985 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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507565 |
Jun 24, 1983 |
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Current U.S.
Class: |
380/218;
348/E7.059; 380/217 |
Current CPC
Class: |
H04N
7/1696 (20130101) |
Current International
Class: |
H04N
7/169 (20060101); H04N 007/167 (); H04N
011/06 () |
Field of
Search: |
;358/120,123,12,114,119 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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642144 |
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Jun 1962 |
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CA |
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750074 |
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Jan 1967 |
|
CA |
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0004083 |
|
Oct 1979 |
|
EP |
|
0021938 |
|
Jan 1981 |
|
EP |
|
0027572 |
|
Apr 1981 |
|
EP |
|
82/01109 |
|
Apr 1982 |
|
WO |
|
83/03942 |
|
Nov 1983 |
|
WO |
|
1252332 |
|
Nov 1971 |
|
GB |
|
1356193 |
|
Jun 1974 |
|
GB |
|
1356970 |
|
Jun 1974 |
|
GB |
|
1382558 |
|
Feb 1975 |
|
GB |
|
1479717 |
|
Jul 1977 |
|
GB |
|
1503051 |
|
Mar 1978 |
|
GB |
|
1521213 |
|
Aug 1978 |
|
GB |
|
1528273 |
|
Oct 1978 |
|
GB |
|
1557741 |
|
Dec 1979 |
|
GB |
|
1602119 |
|
Nov 1981 |
|
GB |
|
Other References
Den Toonder et al, United Kingdom Patent Application No. 2,078,051,
Dec. 23, '81. .
Den Toonder et al, United Kingdom Patent Application No. 2,077,547,
Dec. 16, '81. .
W. Cheung, United Kingdom Patent Application No. 2,042,846, Dec.
24, '80. .
Fondse et al, United Kingdom Patent Application No. 2,038,137, Jul.
16, '80..
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Primary Examiner: Buczinski; Stephen C.
Assistant Examiner: Wallace; Linda J.
Attorney, Agent or Firm: Banner, Birch, McKie &
Beckett
Parent Case Text
REFERENCE TO PRIOR APPLICATION
This application is a continuation-in-part of applicants'
application Ser. No. 507,565 filed June 24, 1983.
Claims
We claim:
1. An apparatus for creating a MAC standard television signal and
for encrypting said MAC signal at the time of its creation, said
apparatus comprising:
input means for receiving a scan line of luminance information, a
scan line of color difference information and an encryption
key;
luminance storage means coupled to said input means for storing a
scan line of said luminance information and subsequently reading
out said stored line of luminance information to create said MAC
standard signal, said luminance storage means including first and
second memory means; and
first and second clock signal means coupled to respective said
first and second memory means, said first clock signal means being
arranged to cause said first memory means to store a present scan
line of said luminance information when said second clock signal
means causes said second memory means to read out said stored scan
line of luminance information, said second clock signal means being
arranged to cause said second memory means to store a present scan
line of said luminance information when said first clock signal
means causes said first memory means to read out said stored scan
line of luminance information, wherein said first and second clock
signal means causes said first and second memory means to delay
reading out said stored scan line of luminance information in
accordance with said encryption key, said first and second clock
signal means causes said respective first and second memory means
to store a predetermined number of samples of said luminance
information at a first predetermined sampling frequency and to read
out sid stored scan line of luminance information at a second
predetermined sampling frequency.
2. The apparatus of claim 1 wherein said predetermined number of
samples is 750, said first predetermined sampling frequency is
14.32 MHz and said second predetermined sampling frequency is 21.48
MHz.
3. The apparatus of claim 1 wherein said first and second memory
means comprises RAM memory.
4. The apparatus of claim 1 wherein said first and second memory
means comprises a plurality of shift registers.
5. The apparatus of claim 1 wherein said first and second memory
means comprises a plurality of CCD elements.
6. The apparatus of claim 1 further comprising color storage means
coupled to said input means for storing a scan line of said color
difference information and subsequently reading out said stored
scan line of color difference information to create said MAC
standard signal.
7. The apparatus of claim 6 wherein said color storage means
includes first and second memory means coupled to said first and
second clock signal means respectively, said first clock signal
means being arranged to cause said first memory means of said color
storage means to store a present scan line of said color difference
information when said second clock signal means causes said second
memory means of said color storage means to read out said stored
scan line of color difference information, said second clock signal
means being arranged to cause said second memory means of said
color storage means to store a present scan line of said color
difference information when said first clock signal means causes
said first memory means of said color storage means to read out
said stored scan line of color difference information, wherein said
first and second clock signal means causes said first and second
memory means of said color storage means to delay reading out said
stored scan line of color difference information in accordance with
said encryption key.
8. The apparatus of claim 7 wherein said first and second clock
signal means causes said respective first and second memory means
of said color storage means to store a predetermined number of
samples of said color difference information at a first
predetermined sampling frequency and to read out said stored scan
line of color difference information at a second predetermined
sampling frequency.
9. The apparatus of claim 8 wherein said predetermined number of
samples is 750, said first predetermined sampling frequency is
14.32 MHz and said second predetermined sampling frequency is 21.48
MHz.
10. An apparatus for creating an NTSC standard television signal
and for decrypting said NTSC signal at the time of its creation,
said apparatus comprising:
input means for receiving a scan line of luminance information, a
scan line of color difference information and a decryption key;
luminance storage means coupled to said input means for storing a
scan line of said luminance information and subsequently reading
out said stored scan line of luminance information to create said
NTSC standard signal, said luminance storage means including first
and second memory means; and
first and second clock signal means coupled to respective said
first and second memory means, said first clock signal means being
arranged to cause said first memory means to store a present scan
line of said luminance information when said second clock signal
means causes said second memory means to read out said stored scan
line of luminance information, said second clock signal means being
arranged to cause said second memory means to store a present scan
line of said luminance information when said first clock signal
means causes said first memory means to read out said stored scan
line of luminance information, wherein said first and second clock
signal means causes said first and second memory means to delay
reading out said stored scan line of luminance information in
accordance with said decryption key, wherein said first and second
clock signal means causes said respective first and second memory
means to store a predetermined number of samples of said luminance
information at a first predetermined sampling frequency and to read
out said stored scan line luminance information at a second
predetermined sampling frequency.
11. The apparatus of claim 10 wherein said predetermined number of
samples is 750, said first predetermined sampling frequency is
21.48 MHz and said second predetermined sampling frequency is 14.32
MHz.
12. The apparatus of claim 10 wherein said first and second memory
means comprises RAM memory.
13. The apparatus of claim 10 wherein said first and second memory
means comprises a plurality of shift registers.
14. The apparatus of claim 10 wherein said first and second memory
means comprises a plurality of CCD elements.
15. The apparatus of claim 10 further comprising color storage
means coupled to said input means for storing a scan line of said
color difference information and subsequently reading out said
stored scan line of color difference information to create said
NTSC standard signal.
16. The apparatus of claim 15 wherein said color storage means
includes first and second memory means coupled to said first and
second clock signal means respectively, said first clock signal
means being arranged to cause said first memory means of said color
storage means to store a present scan line of said color difference
information when said second clock signal means causes said second
memory means of said color storage means to read out said stored
scan line of color difference information, said second clock signal
means being arranged to cause said second memory means of said
color storage means to store a present scan line of said color
difference information when said first clock signal mean causes
said first memory means of said color storage means to read out
said stored scan line of color difference information, wherein said
first and second clock signal means causes said first and second
memory means of said color storage means to delay reading out said
stored scan line of color difference information in accordance with
said decryption key.
17. The apparatus of claim 16 wherein said first and second clock
signal means causes said respective first and second memory means
of said color storage means to store a predetermined number of
samples frequency and to read out said stored scan line of color
difference information at a second predetermined sampling
frequency.
18. The apparatus of claim 17 wherein said predetermined number of
samples is 750, said first predetermined sampling frequency is
21.48 MHz and said second sampling predetermined frequency is 14.32
MHz.
19. A method of simultaneously creating a MAC standard television
signal by compressing luminance or color difference information and
encrypting said MAC standard signal comprising the steps of:
storing a predetermined number of samples of a scan line of
luminance information in one line store at a first predetermined
sampling frequency; and
commencing reading out said stored scan line of luminance
information from said one line store at a second predetermined
sampling frequency after a time delay determined by an encryption
key.
20. The method of claim 19 wherein said predetermined number of
samples is 750, said first predetermined sampling frequency is
14.32 MHz, and said second predetermined sampling frequency is
21.48 MHz.
21. The method of claim 19 further comprising the steps of:
storing a predetermined number of samples of a scan line of color
difference information in a separate line store at said first
predetermined sampling frequency; and
commencing reading out said stored scan line of color difference
information from said separate line store at said second
predetermined sampling frequency after a time delay determined by
said encryption key.
22. A method of simultaneously creating a decrypted NTSC standard
television signal from an encrypted MAC signal by decompressing
luminance or color difference information comprising the steps
of:
storing a predetermined number of samples of a scan line of
luminance information in one line store at a first predetermined
sampling frequency; and
commencing reading out said stored scan line of luminance
information from said one line store at a second predetermined
sampling frequency after a time delay determined by a decryption
key.
23. The method of claim 22 wherein said predetermined number of
samples is 750, said first predetermined sampling frequency is
21.48 MHz, and said second predetermined sampling frequency is
14.32 MHz.
24. The method of claim 22 further comprising the steps of:
storing a predetermined number of samples of a scan line of color
difference information in a separate line store at said first
predetermined sampling frequency; and
commencing reading out said stored scan line of color difference
information from said separate line store at said second
predetermined sampling frequency after a time delay determined by
said decryption key.
Description
BACKGROUND OF THE INVENTION
The present invention relates to the field of television signal
transmission and, more particularly, is directed to a method and
apparatus for creating a television signal and encrypting or
decrypting the signal at the same time.
Television signals are produced and displayed as a result of a line
scanning process. The picture information is scanned using a
progressive series of horizontal lines which are transmitted
sequentially in time. The transmitted signal is a continuous
analogue of the brightness intensity corresponding to each point of
the line. Such a signal is shown in FIG. 1 from which it may be
seen that in a series of standard lines, any two adjacent active
line periods (periods during which video information is
transmitted) are separated by a period in which no video
information is transmitted. This latter period is known as the line
blanking interval and is introduced to allow the scanning device in
the receiver to reset to the line-start position.
In typically color television signals, the active line period
includes one signal which simultaneously represents the
instantaneous values of three independent color components. The
method by which the three color components are coded into one
signal is standardized throughout North America, Canada and Japan.
This method is known as the NTSC standard. Alternative standards
known as PAL and SECAM have been adopted in other countries but
these standards have the same basic format as the NTSC standard,
including a line-blanking interval and an active line period in
each scan line.
Other types of analogue video signals which are particularly
adapted to transmission by satellite and cable, and which lead to
improved picture quality in comparison with existing standards, are
presently being studied. These signals are based on a time
multiplex of the three independent color components during the
active line period of the scan line. Instead of coding the three
components into one signal using the NTSC, PAL or SECAM standard,
the components are sent sequentially using a time-compression
technique. One version of this type of signal is know as MAC
(Multiplexed Analogue Components). Signals generated by a time
comparison technique also adhere to the same basic format as the
NTSC, PAL and SECAM standards, including the presence of a
line-blanking interval and an active line period in each scan line.
It should be noted that when a MAC signal is employed, digital data
may also be transmitted during the line-blanking interval as shown
by the dotted lines in FIGS. 2a and 2c.
Color video signals broadcast under the NTSC standard require that
picture information be separated into two components: luminance, or
brightness, and chrominance, or color. FIG. 10 is an
amplitude-vs.-frequency diagram illustrating, in simplied form, a
typical NTSC composite color television signal 50 comprising a
luminance signal 52 and a chrominance signal 54. (A composite
television signal is one in which chrominance information is
carried on a subcarrier.) The signal occupies a nominal bandwidth
of 6 MHz with the picture carrier 56 being 1.25 MHz above the lower
end of the band. Luminance information is modulated directly onto
picture carrier 56, while chrominance information is modulated onto
color subcarrier 58 which is in turn used to modulate picture
carrier 56. Color subcarrier 58 has a frequency of 3.579545 MHz, a
standard established by the NTSC. (Audio information is carried on
another subcarrier 40 lying near the upper edge of the band.)
The region labeled A in FIG. 10 is of particular importance for it
represents overlap between the luminance 52 and chrominance 54
signals. Since separation of luminance and chrominance is
accomplished by filtering a frequency-division multiplexed signal,
overlaps such as A between the two signals lead to several
problems. If, upon reception, complete separation between luminance
and chrominance is desired, the necessary filtering will cause the
loss of some of the information in both signals. On the other hand,
if no loss of information can be tolerated, then one must accept
interference between the luminance and chrominance signals.
Moreover, since the various parts of the NTSC television signals
are transmitted at different frequencies, phase shifts occurring
during transmission will affect them differently, causing the
signal to deteriorate. Also, the available color information is
severely limited by the small color bandwidth permitted.
As discussed in commonly assigned pending application Ser. No.
652,926 filed Sept. 21, 1984, and herein incorporated by reference,
the above-mentioned MAC standard was developed to overcome the
problems associated with the NTSC standard. A MAC color television
signal is illustrated in FIG. 11, which is an amplitude-vs.-time
diagram of a single video line of 63.56 .mu.s duration. The first
10.9 .mu.s is in the horizontal blanking interval (HPI) 62, in
which no picture information is transmitted. Following HBI 62 are
chrominance signal 64 and luminance signal 66, either of which may
be time-compressed. Between chrominance signal 64 and luminance
signal 66 is a 0.28 .mu.s guard band 68, to assist in preventing
interference between the two signals.
The MAC color television signal of FIG. 11 is obtained by
generating conventional luminance and chrominance signals (as would
be done to obtain a conventional NTSC or other composite color
television signal) and then sampling and storing them separately.
Luminance is sampled at a luminance sampling frequency and stored
in a luminance store, while chrominance is sampled at a chrominance
sampling frequency and stored in a chrominance store. The luminance
or chrominance samples may then be compressed in time by writing
them into the store at their individual sampling frequency and
reading them from the store at a higher frequency. A multiplexer
selects either the luminance store or the chrominance store, at the
appropriate time during the active line period, for reading, thus
creating the MAC signal of FIG. 11. If desired, audio samples may
be transmitted during the HBI; these are multiplexed (and may be
compressed) in the same manner as the video samples. The sample
rate at which all samples occur in the multiplexed MAC signal is
called the MAC sampling frequency.
Although the MAC format of FIG. 11 overcomes the problems of the
composite television signal of FIGS. 1 and 10, there also exists in
the prior art a need for secure encryption of video signals, such
that only designated users may decrypt and display the information.
In typical encryption systems, one or more parameters of the signal
to be encrypted are modified according to a pattern which is
determined at the transmitter. The pattern generally is a member of
a large class of similar patterns, such that discovery of the
pattern through exhaustive search is extremely unlikely. A precise
description of the pattern used for encryption is delivered to
designated receivers which then are able to recover the original
information. The description of the pattern is known in the art as
the "encryption key" and the process of informing designated users
of the encrytion key is known as "key distribution."
With reference to FIG. 1, various encryption techniques known in
the art will be described. As shown in FIG. 1, the video signal
during the active line period may be represented by:
where
y=amplitude (voltage) and
t=time
Knowledge of both the signal's amplitude (y) and the time at which
it occurs (t) is necessary for accurate reconstruction of the video
signal in a line scan system.
Encryption techniques may be classified as follows:
(1) Those which modify the amplitude (y) of the transmitted signal
according to a prescribed pattern.
where
f=f (t)
Examples of this technique include amplitude reversal of randomly
chosen lines:
(2) Those which modify the time at which the signal is transmitted
through the channel:
Examples of this technique include the reordering of television
lines according to a prescribed pattern:
(3) Those which modify both amplitude and transmission time.
It has been found that encryption techniques from the first
category (variation of amplitude) cause distortion when the channel
through which the signal is to be passed is non-linear. In this
case, an amplitude (y) will be represented in the scrambled channel
by various amplitudes according to the scrambling function in use
at that instant. Channel non-linearity, therefore, causes imperfect
reconstruction of the video information at the receiver. Since
amplitude non-linearity is very common, it has been found that an
optimum encryption algorithm should be selected from the second
category, and, in particular, from the subset:
where d is constant during each standard line. In this case, the
channel is subjected to an undistorted signal and only the time at
which the signal occurs is scrambled. Since almost all channels are
essentially `time invariant,` this technique introduces little
distortion. This system is known as time-base scrambling.
An obvious method of time-base scrambling which has been used, is
to reorder the television lines within the picture. This method,
which results when d in the previous equation is an integral number
of line periods, is complex, expensive and difficult to implement
because recovery of the picture in the receiver demands storage of
many television lines.
SUMMARY OF THE INVENTION
It is, therefore, the overall object of the present invention to
provide a method and apparatus for creating a television signal
while at the same time encrypting and decrypting the signal.
It is a specific object of the present invention to provide a
method and apparatus for time-base scrambling of television signals
which is relatively simple and which can be readily
implemented.
It is another specific object of the present invention to provide a
method and apparatus for time-base scrambling of television signals
which can be implemented at low cost while at the same time being
reliable in operation.
It is a still further specific object of the present invention to
provide a method and apparatus for time-base scrambling of
television signals which requires storage of only a very small
number of television lines in the receiver.
It is another specific object of the present invention to provide a
method and apparatus for creating an encrypted MAC standard
television signal for transmission and for creating a decrypted
NTSC standard television signal for display on a television
receiver.
These and other objects of the present invention are achieved by
using the same apparatus to create a television signal and to
encrypt and decrypt the signal at the same time. In accordance with
the present invention, a MAC standard television signal may be
created and encrypted for transmission to a remote receiver. At the
receiver end, the MAC signal may be used to create a decrypted
signal, as for example an NTSC signal, for display on a television
receiver. The MAC signal is created at the transmitter end by
sampling and storing the luminance and chrominance signals
separately. Luminance is sampled at a luminance sampling frequency
and stored in a luminance store while chrominance is sampled at a
chrominance sampling frequency and stored in a chrominance store.
The luminance and chrominance samples are compressed in time by
writing them into the store at their individual sampling frequency
and reading them from the store at a higher frequency. A
multiplexer selects either the luminance store or the chrominance
store, at the appropriate time during the active period of the
video scan line, for reading, thus creating the MAC signal. The
signal may be encrypted by varying the starting time at which the
luminance and/or chrominance signals are read out from their
respective stores in accordance with an encryption key.
At the remote or receiver end, a decrypted signal, e.g., an NTSC
signal, may be created for display on a television receiver using
the same method and apparatus as used to create the encrypted
signal at the television transmitter end. This is accomplished by
also storing the incoming luminance and chrominance signals in
individual stores. The signals are read out from the stores at a
frequency corresponding to the desired format, i.e., the NTSC
standard. The signal is decrypted by varying the starting time at
which the luminance and/or chrominance signals are read out from
their respective stores in accordance with a decryption key.
Thus, the method and apparatus of the present invention may be used
for creating an encrypted television signal for transmission to a
remote receiver and for creating a decrypted signal at the receiver
for display. Accordingly, a television broadcast system which
embodies the present invention uses fewer component parts, is
simplier in construction, more reliable in operation and is lower
in cost.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a standard NTSC television signal;
FIGS. 2a, 2b and 2c illustrate the encryption technique employed in
the present invention;
FIG. 3 shows one form of an encryption/decryption system which may
be used with the present invention;
FIG. 4 shows an alternative system to that shown in FIG. 3;
FIG. 5 illustrates another embodiment of an encryption system which
may be used with the present invention;
FIGS. 6 and 7 illustrate two different decryption systems which may
be used with the present invention;
FIG. 8 shows a decryption system embodying an aspect of the
invention and surrounding equipment;
FIG. 9 illustrates apparatus that may be employed for the
encryption and decryption of video signals by a technique embodying
the present invention;
FIG. 10 is an amplitude-vs.-frequency diagram illustrating in
simplified form a typical NTSC color television signal;
FIG. 11 is an amplitude-vs.-time diagram of a single video line of
a typical MAC color television signal;
FIG. 12 is a block diagram of a line store which may be used to
compress or decompress television scan lines in accordance with the
present invention;
FIG. 13 is a block diagram of the clock signals used to control the
line store shown in FIG. 12;
FIG. 14 is a block diagram of an encoder which may be used with the
present invention;
FIG. 15 is a block diagram of a decoder which may be used with the
present invention; and
FIGS. 16a and 16b are diagrams illustrating the signals input to
and output from the line store of FIG. 12.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The encryption and decryption method of the present invention is
based on the derivation and use of a variable scan line period as
shown in FIGS. 2a-2c. Referring to FIG. 2a, portions of the active
video components of lines N and N+1 are shown along with the
line-blanking interval of line N+1. The line shown in FIG. 2a is of
standard length and thus includes a standard line-blanking
interval. As discussed previously, and as shown in dotted outline
in FIG. 2a, instead of there being a line-blanking interval, there
may be a period of standard length for transmission of digital
data.
A line of minimum length is shown in FIG. 2b and is obtained by
virtually eliminating the standard line-blanking interval or the
period of digital data transmission.
A line of extended length is shown in FIG. 2c and is obtained by
increasing the standard line-blanking interval or the period of
digital data transmission shown in FIG. 2a, the dotted outline in
FIG. 2c also indicating digital data.
An extended length line of the type shown in FIG. 2c can be derived
with simple hardware in the case where the line-blanking interval
is double the line-blanking interval of FIG. 2a. In fact, the
extended length line of FIG. 2c is such a line and has twice the
line blanking interval of the standard line of FIG. 2a.
Encryption is achieved according to the present invention by
varying the line-blanking intervals of some of the lines to derive
minimum and extended length lines. The transmitted television
signal is then composed of lines of all three different lengths in
accordance with an encryption key.
It will be appreciated that over some specified period of time it
is necessary for the average line length to be equal to the length
of a standard line, i.e., that the long and short lines must cancel
or balance each other out. This period is not critical. It may be
one field, for example, or one frame, or it may be even a longer
period. The longer this period is, however, the longer it will take
for the receiver to lock in on the signal.
While FIGS. 2a-2c illustrate an embodiment of the invention where
the line-blanking interval is standard, zero and two times
standard, line-blanking intervals between zero and standard can be
employed as well as line-blanking intervals more than twice
standard and/or between one and two times standard. There may also
be a number of different line-blanking intervals greater than
standard. Generally speaking, however, employing a standard and
more than two other line blanking intervals can be done only at the
expense of more sophisticated hardware.
In another embodiment of the invention, no standard length line is
employed, i.e., the line-blanking intervals of all lines are
lengthened or shortened. Thus, in the practice of the present
invention a television signal is modified in accordance with an
encryption key to produce a signal in which all active video lines
are transmitted unchanged except for a time delay equal to the
accumulated variance in the line-blanking periods. More
specifically, and as determined by the encryption key, some lines
may be left with unchanged line-blanking intervals, the
line-blanking intervals of other lines are increased and the
line-blanking intervals of still other lines are decreased. The
encrypted television signal is composed of all of these lines and
is what is transmitted, the encryption key indicating which lines
are standard lines, which lines are long lines and which lines are
short lines to enable decryption of the received signal.
One additional condition is require to ensure a low-cost receiver.
This condition is that the accumulated change of the line-blanking
periods at any given time should remain within the range of from
0-1 line. With this constraint, the lines which arrive at the
receiver do not require more than one line of delay before they are
used in reconstructing the original signal, i.e., the signal prior
to encryption. It is to be understood clearly, however, that this
is not a limitation of the present invention. If the accumulated
change in the line-blanking periods at any given time will be more
than one line, all that is required is to ensure that apparatus
capable of storing the accumulated change is available. This
requirement simply introduces greater cost and complexity.
Because certain of the line-blanking periods have been completely
or partially removed, it is necessary to regenerate the blanking
waveforms in the receiver. This can be achieved simply using
electronic memories. More specifically, in the case of an NTSC
signal, for example, regeneration of the line-blanking intervals
will require regeneration of the line synchronizing signals and the
color burst signals. This can be done using prior art techniques,
however, and is not a part of the present invention. Thus, once the
decryption key, which is the same as the encryption key, has been
employed to restore the active video components to their proper
time relationship with respect to each other, sync and color burst
signals correctly timed with respect to the video signals can be
added readily and by known means.
In the case where digital data is present during what would
otherwise appear to be a line-blanking interval, it might appear
from FIG. 2b that the digital data would be lost by the practice of
this invention. The data is not lost, however, but rather is
transmitted during longer than standard digital data periods, as
shown in FIG. 2c, for example.
The encryption/decryption technique described herein can be
implemented in a large number of ways using known techniques,
equipment and components. Thus, referring to FIG. 3, for example,
the television signal produced by TV camera 12 is supplied to an
optional analogue to digital converter (ADC) 13, the digital output
of which is supplied to a line storage device 14. The output of
line storage device 14 is supplied to an optional digital to
analogue converter (DAC) 15 whose output, which is an encrypted
television signal in analogue form, is supplied to a transmitter 16
for broadcast to a satellite 17, for example. An encryption key for
encrypting the television signal in line storage device 14 is
supplied to encoding and timing networks which vary the
line-blanking intervals of the television signal.
The encrypted signal is received by a cable head end receiver 19
and is supplied to an optional ADC 20 whose digital output is
supplied to a line storage device 21. The output of line storage
device 21 is supplied to an optional DAC 22 whose output, which is
a decrypted TV signal the same in all respects as that derived at
the output of camera 12, is supplied via cable to cable
subscribers. A decryption key, which is the same as the encryption
key, for decrypting the television signal in line storage device 21
is supplied to decoding and timing networks 23 which restores the
shortened and extended link-blanking intervals to the standard
length shown in FIG. 2a.
In the case where the TV signal is an NTSC signal, for example, it
may be necessary to restore line and field synchronizing signals
and color burst signals. This function is performed by blanking
interval regenerating network 24.
The TV signal may be processed in either analogue or digital form.
The nature of line storage devices 14 and 21 will depend upon the
format of the signal. Thus, if the TV signal is in analogue form,
line storage devices 14 and 21 may be so-called bucket-brigade
devices, while, if the TV signal is in digital form, line storage
devices 14 and 21 may be shift registers or may be RAM with at
least one line memory capacity or CCD storage devices.
FIG. 8 shows a decryption system embodying the present invention in
somewhat greater detail.
FIG. 9 shows how the decryption (or encryption) key is used to vary
the line lengths. The decryption key (which in the embodiment
shown, is updated once a frame) is used as a starting vector for a
pseudo-random number generator circuit. This circuit produces (for
the NTSC standard) a sequence of 525 random numbers based on the
decryption key. These random numbers then are combined with
information derived from a counter, which is incremented once per
line, in a line type selection circuit. This circuit selects the
type of line (i.e., determines the length of the blanking interval)
for the next line. This information is then fed to the line length
controller which monitors the aggregate deviation in line lengths
referenced to the start of the current frame and ensures that for
this particular embodiment the following two conditions are
met:
1. The aggregate deviation never exceeds one full video line
(63.56.mu. sec for an NTSC signal);
2. The aggregate deviation at the end of the frame is zero.
The line controller then provides information to the horizontal
counter and its associated decoder which enables this
counter/decoder to produce the correct line store control signals
for the current line.
As pointed out above, use of the MAC standard for transmission of
television signals eliminates many of the problems associated with
the NTSC standard. FIG. 12 is a block diagram of a line store which
may be used to compress or decompress luminance and chrominance
signals to create a MAC standard television signal. The store
comprises a pair of memory elements 33 and 34 coupled to a common
input 35 which recieves either luminance or chrominance, i.e.,
color different signals. Memory elements 33 and 34 may be selected
from among a number of memory elements known in the art and are
shown in FIG. 12 as being CCD memory elements. Memory elements 33
and 34 are coupled to respective clock signals 30 and 31 and to
selector switch 36. Switch 36 is an electronic switch or
multiplexer well known in the art and which has a
double-pole-single-throw (DPST) function. Each respective output
line of memory elements 33 and 34 are coupled to switch 36 and
selectively passed to output line 37 as controlled by output select
signal 32.
Though the line store of FIG. 12 may be used to both compress and
decompress signals, the device is described hereafter as performing
compression. When a signal, as for example a luminance signal,
arrises at input 35, clock 30 writes a predetermined number of
luminance samples into memory element 33 at a predetermined
incoming sampling frequency. It has been found that a suitable
number of samples is 750 and that a suitable incoming sampling
frequency is 14.32 MHz for a luminance signal in accordance with,
for example, the NTSC standard. At the same time that memory
element 33 is storing the incoming luminance signal, clock 31 is
causing the contents of memory element 34 (luminance signals from
the previous scan line) to be read onto output line 37 through
switch 36 at a predetermined outgoing sampling frequency. It has
been found that a suitable outgoing sampling frequency is 21.48
MHz. During the next scan line, the 750 luminance samples are
written into memory element 34 by clock 31, operating at the
incoming sampling frequency of 14.32 MHz. At the same time, the
luminance samples stored in memory element 33 are read out to
output line 37 by clock 30 at the outgoing sampling frequency of
21.48 MHz. A separate line store having a pair of memory elements
is used to compress the color difference signals (i.e., chrominance
signals) and operates in a similar manner. FIG. 13 is a block
diagram of the clock signals used to control the operation of
memory elements 33 and 34.
FIG. 14 is a block diagram of an encoder which may be used with the
present invention and includes the line store shown in FIG. 12 for
storing and subsequently reading out the luminance and chrominance
signals. As shown, three color television signals, luminance (Y)
and two color difference signals (R-Y and B-Y) are delivered from a
television signal source and are filtered, respectively, in
low-pass filters 100a, 100b and 100c. The filtered signals are then
sampled at the appropriate incoming sampling frequency in A/D
converters 102a, 102b, and 102c.
Vertical filters 104 and 106 provide vertical interpolation of the
digital color difference signals R-Y and B-Y, respectively, after
which these signals are selected alternately for transmission by
multiplexer 108. Only one of the two color difference signals need
be sent as chrominance in each line in order to create a MAC
television signal.
The digital luminance and chrominance signals are next compressed
as described above. Luminance data are written into and read from
luminance store 38a. Chrominance data are written into and read
from chrominance store 38b.
Multiplexer 118 receives four sets of signals, luminance,
chrominance, audio, and synchronization. Multiplexer 118 then
combines these signals by selecting them at the appropriate time
for inclusion in the MAC video line. After multiplexing, the
signals are reconverted to analog in D/A converter 120, filtered in
low-passs filter 122, and output as a MAC color television
signal.
FIG. 15 is a block diagram of a decoder which may be used with the
present invention and also includes the line stores shown in FIG.
12. The incoming television signal first enters the demultiplexer
300, which separates from it the luminance and chrominance signals
as well as the audio and synchronization signals. The luminance
signal is delivered to luminance store 38a where it is
decompressed, and then to low-pass filter 304, where it is
filtered. The analog luminance signal then goes to output interface
306. The sampling signals necessary to decompress luminance are
produced in timing generator 308 and supplied to luminance store
38a by two clock drivers 310.
The chrominance signal from demultiplexer 300 is also decompressed
in chrominance store 38b. Separate outputs are provided for the two
color difference signals, which are filtered in two low-pass
filters 314 and then supplied to output interface 306. The
necessary sampling signals are supplied to chrominance store 38b
from timing generator 308 through three clock drivers 310.
Signals not constituting luminance or chrominance are also
separated from the incoming television signal by demultiplexer 300.
Output interface 306 receives luminance from low-pass filter 304,
chrominance from low-pass filters 314, and timing signals from
timing generator 308. Its output is a standard NTSC color
television signal.
In accordance with the present invention, the line store of FIG. 12
may also be used to encrypt the television signal at the
transmitter end during the creation of a MAC signal and to decrypt
the created signal at the receiver end for display on a television
receiver. Thus, the line store of FIG. 12 may be used to replace
stores 14 and 21 shown in FIG. 3 to create an encrypted signal with
respect to store 14 and to create a decrypted signal with respect
to store 21. As shown in FIG. 9, an encryption/decryption key is
supplied to networks 23 which controls the generation of control
signals for the line store. These signals include clock signals 30
and 31 and output select signal 32. Depending on the type of
memories used for the memory elements within the line store, the
control signals may also include read, write and memory refresh
signals.
FIGS. 16a and 16b are diagrams illustrating the signals input and
output from the line store of FIG. 12. To simplify the illustration
from the line store of FIG. 12. To simplify the illustration and
explanation, it is assumed that the line store is being used to
compress a luminance signal, that the length of a standard scan
line L is 64 .mu.s and that the length of the horizontal blanking
interval is 12 .mu.s. Thus the length of the active or video
portion of the scan line is 54 .mu.s. FIG. 16a represents the
operation of memory element 33 while FIG. 16b represents the
operation of memory element 34.
As shown in FIG. 16a, 750 samples of luminance are taken at a
sampling frequency of 14.32 MHz over 52 .mu.s of the scan line and
stored in memory element 33. After a delay in accordance with the
encryption key, these samples ae read out at a sampling frequency
of 21.48 MHz. The higher sampling frequency causes the samples to
be read out much faster than they were read in, i.e., the incoming
samples took 52 .mu.s to read in but only 34 .mu.s to read out.
Therefore, compression of the incoming luminance singal is
accomplished, thereby creating a luminance portion of MAC standard
signals. Because readout of the samples is delayed in accordance
with an encryption key, the MAC signal is also simultaneously
encrypted. As can be seen from FIG. 16a, the length of the delay
before reading out the stored samples can vary from 0 to 32 .mu.s.
A delay longer than 32 .mu.s will not permit the samples to be
completely read out prior to the arrival of a new scan line for
storage.
Memory elements 33 and 34 operate in tandem, while one memory
element is reading in luminance samples, the other memory element
is reading out luminance samples from the previous scan line in MAC
standard format. Thus, each memory element operates on alternate
scan lines.
Likewise the line store of FIG. 12 could be used to decrypt the
signal at the receiver while simultaneously decompressing the
signal. In that event, the luminance signal will be read into the
line store at 21.48 MHz and read out at 14.32 MHz to decompress the
signal. The readout time can be controlled in accordance with a
decryption key to simultaneously decrypt the signal.
It will be understood that cable distribution of the TV signal
after decryption is not essential to this invention. FIG. 4
discloses an arrangement whereby encrypted signals are received by
an antenna 26 at a user location, e.g., a home, decrypted at that
location and supplied to a TV receiver 25 at the location.
One form of a decryption system that can be used in practicing the
present invention is shown in FIG. 6, line storage device 21 in
this case being a one line RAM memory. Components 27 and 28 simply
are low pass filters. With the system of FIG. 6, read and write
cycles occur independently during each TV line.
Another form of a decryption system that can be used in practicing
the present invention is shown in FIG. 7, storage device 21 in this
case being a number of shift registers. With the system of FIG. 7,
the read-in and read-out cycles occur on different TV lines. The
system of FIG. 7 also can be implemented using CCD technology.
An encryption system of a type paralleling the decryption system of
FIG. 6 is shown in FIG. 5. Obviously an encryption system
paralleling the decryption system of FIG. 7 also could be used and
would be identical to that shown in FIG. 5 but with storage device
14 thereof being a plurality of shift registers connected as shown
in FIG. 7.
While a preferred embodiment of the present invention has been
described and illustrated herein, a person skilled in the art will
appreciate that changes and modifications may be made therein
without departing from the spirit and scope of this invention as
defined in the appended claims.
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